The present invention relates to improvements to energy absorbing guardrail systems having end terminals, anchor cable release mechanisms, and breakaway posts used in cooperation with longitudinal, W-beam guardrail sectional barriers. These systems usually extend along highways and roadsides to absorb impact energy and deflect vehicles from hazards which may be associated behind the barriers. The present invention more specifically relates to systems having sequential kinking terminals (SKT) and flared energy absorbing terminals (FLEAT). More particularly, the present invention relates to an improved feed chute shield for the terminal; improved quick anchor cable release mechanisms; an improved breakaway post (Post 1) which facilitates breakaway in head-on impacts while resisting loads on side impacts; and an improved anchorage system that maintains tension in the W-beam rail after initial release of tension due to cable anchor release in order to reduce the propensity for the W-beam rail to buckle and form an elbow that may pose a hazard to the impacting vehicle. Each of these improvements may be incorporated into existing energy absorbing guardrail systems, alone or in combination, to improve the overall safety performance of the systems.
Impact heads of existing SKT, FLEAT, and other energy absorbing terminals do not have a shield to protect traffic-side exposure to the W-beam rail guide tube or feeder chute. For angled impacts in the area of the feeder chute, an impacting vehicle may potentially wedge into the opening of the existing prior art feeder chute. Such wedging may possibly cause the impacting vehicle to get hung up, thus, preventing smooth redirection of the vehicle. Wedging also may potentially snag vehicle parts in situations where it should be an easy gate-through. Such wedging, in turn, could lead to rollover of the impacting vehicle. Further, in the existing prior art feeder chute, the W-beam rail may buckle out of the traffic-side of the chute as the impact terminal head and the feeder chute are urged downstream by the impacting vehicle. When such buckling occurs the entire energy absorption process may stop.
An embodiment of the present invention provides a shield plate extending along the traffic-side of the chute substantially the entire length of the chute. This shield plate closes the traffic-side of the feeder chute and prevents impacting vehicles from wedging into the feeder chute. The closing shield also prevents the W-beam rail from buckling out the traffic-side of the chute as it is urged downstream along the W-beam rail element.
In the past either a heavy strap has been wrapped at the end of the feed chute or a heavier chute wall thickness is used to prevent cutting of the chute and/or W-beam railing as the chute travels downstream along the top and bottom edges of the W-beam upon vehicular impact. It has been discovered that utilizing inwardly rolled edges at the downstream end of the feed chute reinforces the chute and prevents such cutting as the chute travels downstream along the W-beam.
Existing SKT and FLEAT terminals depend on the break away of Post 1 to release the upstream end of an anchor cable. However, under certain impact conditions, Post 1 may not break away properly, thus not releasing the anchor cable. This in turn may result in snagging and excessively high deceleration of the impacting vehicle.
An embodiment of the present invention provides for the release of the anchor cable at the downstream end (i.e., at the anchor release bracket) rather than relying on the breaking away of Post 1 to release the upstream end of the anchor cable. The improved anchor cable release mechanism includes a release arm attached to the anchor cable release bracket with a pivot bolt and alignment shear pins to release the anchor cable at the downstream end of the cable.
In another embodiment, a plurality of the improved anchor cable release brackets may be mounted to downstream sections of the guardrail with additional cable lengths swagged together to span from Post 1, through the first anchor release bracket, to the subsequent downstream anchor brackets. The upstream end of the anchor cable is attached permanently to Post 1. While the present disclosure discusses a system with two such anchor cable release brackets, it should be understood that more such brackets may be utilized to maintain tension in the W-beam rail element as the impact head is urged downstream on impact.
In a typical end-on impact with a single anchor cable release bracket, once the impacting vehicle pushes the impact head downstream, breaking away Post 1, and releasing the anchor cable from the first anchor cable release bracket and pushing the first release bracket off the W-beam rail, the tension in the W-beam rail is released. With the two (or more) anchor release bracket embodiment of the present invention, after the anchor cable is released from the first anchor bracket and the first bracket is pushed off the W-beam rail, the tension in the W-beam rail is maintained by the second (or other) anchor cable release brackets. The rail tension maintained through the release of subsequent brackets reduces the propensity for the W-beam rail to buckle and form an elbow that may pose a hazard to the impacting vehicle. Thus, the rail tension is maintained until the impact head releases the subsequent anchor brackets and releases the downstream-most end of the anchor cable.
In an embodiment of the present invention, a supplemental anchor cable mechanism is provided to maintain tension in the W-beam rail after release of the primary anchor cable. The supplemental anchor cable system is designed to reduce the propensity of the W-beam rail to buckle in end-on impact at an angle.
An additional and separate anchor for the supplemental anchor cable mechanism may be installed upstream of the impact head. In yet another embodiment, this supplemental mechanism is incorporated into the Post 1 anchor as will be described below. The supplemental anchor cable may be attached to the additional anchor at its upstream end, extend through the impact head, and may be retained by a bracket attached to an upstream end of the W-beam rail. Sufficient slack is provided in the supplemental anchor cable length so that the supplemental cable is not tensioned until it becomes taut.
Testing of end-on impacts shows that after the primary anchor cable is released from the cable anchor release mechanism, tension in the W-beam rail is released until the supplemental anchor cable becomes taut. At that point, tension in the W-beam rail is re-established by the supplemental anchor cable system.
A feature of the prior art Post 1 design is that Post 1 is intended to breakaway when the post is impacted from a head-on direction, but the post has limited lateral strength. Thus, for side impacts just downstream of Post 1, the prior art Post 1 design may unintentionally break away allowing the impacting vehicle to gate through the terminal and go behind the guardrail installation. An embodiment of the present invention provides for an improved post design that still allows Post 1 to break away in head-on impact, while providing added lateral strength to accommodate side impacts just downstream of Post 1.
An alternative embodiment of the present invention utilizes the anchor cable release bracket disclosed and claimed in U.S. Pat. No. 8,448,913, but utilizes an improved upstream anchor cable release mechanism.